Unidirectional surface acoustic wave transducer

Abstract

A low-loss, wideband SAW transducer operating on a piezoelectric substrate with a longitudinal axis of propagation. The SAW transducer has a plurality of tapered interdigital electrodes that are mounted on the substrate transverse to the propagation axis. The electrode widths and spacings are constant along lines parallel to the axis and are tapered in the direction transverse to the axis. An acoustic-wave reflector having a plurality of elongated, tapered discontinuities or reflective fingers is located on the substrate adjacent one end of the transducer. The spacings and widths of the reflective fingers are similar to those of the electrodes, i.e., the finger widths and spacings are tapered in the direction transverse to the axis and are constant along lines parallel to the axis. Because of the taper, the fingers reflect wideband acoustic waves parallel to the axis.

What is claimed is: 
     
       1. A SAW structure comprising: a piezoelectric substrate having an axis of propagation;   a SAW transducer having a plurality of interdigital, quarter-wave electrodes mounted on the substrate transverse to the axis with an electrode spacing on one side of the axis being different from an electrode spacing on the other side of the axis, the widths of and spacing between the electrodes tapering in a direction transverse to the axis, and wherein the transducer includes a first and a second bus bars connected to alternate ones of the interdigital electrodes; and   acoustic-wave reflector means mounted on the substrate adjacent one end of the transducer for reflecting acoustic waves propagating on the substrate in a first direction which is transverse to the electrodes to a second direction which is substantially opposite to the first direction, wherein the acoustic-wave reflector means includes a discontinuity grating which is aligned on the substrate transverse to the axis and such that surface acoustic waves reflected by the reflector means constructively add to acoustic waves propagating on the substrate in the second direction and wherein said discontinuity grating has spaced reflective fingers having spacings that are different on opposite sides of the axis.   
     
     
       2. The SAW structure of claim 1 wherein said spacings of said fingers taper in a direction transverse to said axis. 
     
     
       3. The SAW structure of claim 2 wherein said electrode spacing and said finger spacing are substantially one-quarter the acoustic wavelength of incident waves propagating parallel to said axis. 
     
     
       4. A SAW structure comprising: a piezoelectric substrate having an axis of propagation;   a tapered SAW transducer having a plurality of interdigital, quarter-wave electrodes mounted on the substrate transverse to the axis with the width and spacing of the electrodes tapered, the transducer including a first and a second bus bars connected to alternate ones of the interdigital electrodes; and   acoustic-wave reflector means mounted on the substrate adjacent one end of the transducer for reflecting acoustic waves propagating on the substrate in a first direction, which is transverse to the electrodes, to a second direction, which is substantially opposite to the first direction, wherein the acoustic-wave reflector means includes a discontinuity grating which is aligned on the substrate transverse to the axis and such that surface acoustic waves reflected thereby constructively add to acoustic waves propagating on the substrate in the second direction and wherein said discontinuity grating has spaced reflective fingers having tapered widths and spacings.   
     
     
       5. The SAW structure of claim 4 wherein the spacings of said electrodes and said fingers are substantially one-quarter the acoustic wavelength of incident waves propagating parallel to said axis.